New research

New study directly measures greenhouse effect at Earth’s surface

  • 25 Feb 2015, 18:00
  • Robert McSweeney

Scientists know that greenhouse gases in the atmosphere cause the Earth to warm. But measuring exactly how much heat they trap is harder than you might think.

Previous studies using satellites have established that more heat is entering the atmosphere than leaving it. But a new study goes a step further and directly measures the amount of warming greenhouse gases are producing at Earth's surface.

The paper provides the critical link between rising carbon dioxide concentrations and the extra energy trapped in the climate system, the researchers say.

Greenhouse effect

Joseph Fourier first suggested in the 1820s that gases in the Earth's atmosphere trap heat and help keep the planet warm, coining the term greenhouse effect. Physicist John Tyndall later extended the theory by identifying the gases, such as carbon dioxide and methane, that were responsible for the warming.

Jumping forward a century and a half, we now know a lot more. Using satellites to measure how much of the sun's energy enters the Earth's atmosphere, and how much is reflected or re-emitted back into space, scientists have shown that the difference between the two is increasing. This means the Earth is trapping more heat than it used to, and therefore must be warming.

But while those studies show a widening gap between the energy reaching and leaving Earth, they are unable to directly measure how much warming greenhouse gases are causing at a particular point in time. New research, published today in Nature, shows how scientists have directly been able to measure the warming effect of greenhouse gases at Earth's surface.

Measuring energy

The researchers used a set of instruments to take thousands of measurements at the Earth's surface. The instruments record the longwave energy that is re-emitted by greenhouse gases back towards the Earth's surface, which causes the warming.

Making these sorts of measurements on the ground is difficult, says lead author Dr Daniel Feldman, a geological scientist at the Lawrence Berkeley National Laboratory in the US. With weather systems passing overhead, and temperatures and humidity changing frequently, it's tricky to take energy measurements without other factors getting in the way.

To overcome this problem, the researchers measured temperature and water vapour at the same locations so that their influence on warming could be eliminated from the calculations, leaving just the impact of greenhouse gases.

The scientists used data from 2000 to 2010, collected from two sites in the US: the southern Great Plains and northern Alaska. They chose these sites because of their very different climates, says Feldman. This meant the researchers could investigate both a mid-latitude and a high-latitude location.

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Surface warming 'hiatus' could stick around another five years, say scientists

  • 23 Feb 2015, 16:30
  • Roz Pidcock

Don't be surprised if the slower pace of warming we're seeing at the Earth's surface lasts for another five years, scientists say.

new paper out today puts the chances of the so-called "hiatus" staying until the end of the decade at about 15 per cent, or one in six.

But the heat hasn't gone away. The scientists say most of it is lurking in the deep ocean and we can expect the pace of warming to pick up when this heat gets released again.

Slower surface warming

Since 2000, the temperature at the Earth's surface  hasn't warmed as quickly as it has in previous decades, despite greenhouse gas emissions rising  faster than they were before.

A growing body of evidence is  homing in on the  Pacific Ocean as the main culprit for why we're seeing "unexpectedly modest" warming, as the Nature Climate Change paper puts it.

Scientists think a natural fluctuation is causing heat to find its way to the deep ocean in the Pacific, where it doesn't warm the atmosphere as much it would if it stayed at the surface.

A number of recent studies have found that periods of faster and slower warming  aren't unusual in Earth's temperature record. It's what scientists expect as these natural cycles flip-flop between their  different phases, superimposed on top of greenhouse gas warming.

But what are the chances of natural variability being strong enough to offset some, or even all of the warming expected from greenhouse gases?

The new paper by Dr Chris Roberts, an ocean and climate specialist at the Met Office Hadley Centre, and colleagues at the University of Exeter sheds some new light on this question.

Odds of a 'hiatus'

The new paper uses a suite of climate models to examine past temperatures with and without greenhouse gas forcing. The authors find there's a 28 per cent chance natural variability could cause a five-year long 'hiatus'.

The scientists define 'hiatus' as a period during which the observed temperature rise is less than the warming expected from greenhouse gases of 0.2 degrees Celsius per decade.

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Uncertainty behind climate projections could be cut in half by 2030, study shows

  • 23 Feb 2015, 16:00
  • Robert McSweeney

Scientists will soon be able to forecast climate change more accurately, according to new research. Projections of future temperature rely on estimates of how sensitive the Earth's climate is to rising emissions, and the uncertainty in those estimates could be halved within 15 years.

More certainty about the climate's sensitivity to emissions means a better assessment of our chances of keeping global temperature rise below the two-degree limit, the researchers say.

Climate sensitivity

Climate sensitivity is the amount of warming we can expect when carbon dioxide in the atmosphere reaches double the level before the industrial revolution. On current emission trends, we're set to reach that point shortly after 2050.

There are two ways to express climate sensitivity. Equilibrium Climate Sensitivity (ECS) refers to the total amount of warming once the Earth has had time to adjust fully to the extra carbon dioxide. ECS allows for 'feedbacks' in the climate system that can amplify or slow the pace of warming, many of which act over decades or even centuries.

An alternative option is the Transient Climate Response (TCR), which is the warming at earth's surface we can expect at the point of doubling. This doesn't take into account long term feedbacks, and so estimates of TCR are lower than for ECS.

In its 2013 report, the IPCC estimates TCR is likely to lie between 1.0 and 2.5 degrees Celsius. The new research, published in Nature Geoscience, suggests scientists will be able to reduce the uncertainty around these estimates by about 50 per cent by 2030.

Bigger proportion

The new paper deals with one way to estimate TCR, which is to compare how much greenhouse gases have risen over the industrial period with observations of how much the temperature has changed in that time.

But factors such as aerosols and other greenhouse gases have contributed to the observed temperature change, making it difficult to calculate TCR from historical observations.

Aerosols are released into the atmosphere when fossil fuels are burned. These tiny particles have a direct effect on temperature by scattering sunlight, and an indirect effect by stimulating cloud formation, preventing sunlight reaching Earth's surface.

Although scientists know that aerosols have an overall cooling effect on the climate, they aren't as certain about the size of the temperature effect as they are for carbon dioxide.

But uncertainty over aerosols is set to be less of a problem in the near future, the study says.

While emissions of carbon dioxide are expected to rise in the next few decades, emissions of aerosols and other greenhouse gases are expected to slow, or fall. This means carbon dioxide will make up a bigger proportion of the human-caused factors affecting the climate.

You can see this in the graph below: the influence of carbon dioxide on the climate (red line) is projected to increase more rapidly than aerosols and other gases over the next 15 years (blue and green lines).

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